1. Field of the Invention
The invention relates to the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425 C.-600 C., usually 460 C.-560 C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500 C.-900 C., usually 600 C.-750 C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. The single dense bed usually flows in either a swirl pattern, or in a crossflow pattern. These units work, but at relatively low efficiency compared to to more modern designs. The older units have had problems in establishing a desired gas flow through the bed, or were considered inefficient because they maintained the catalyst as a "bubbling" dense phase fluidized bed. Bubbling dense beds have never worked as will in large refinery units as they do in pilot plant size units. Much of the deficiency in operation was laid to the presence of large bubbles in the bed, which meant that the dense phase fluidized bed was not being efficiently used much of the time.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units of this type are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many other recent patents. The H.E.R. design is used in most new units because it permits operation of an FCC with less catalyst inventory (and hence less catalyst loss), and because such units tend to have both less CO emissions and less NOx emissions than the single dense bed regenerators.
The high efficiency design uses a fast fluidized dense bed for coke combustion. These dense bed are intensely agitated, and large bubbles are not stable in such beds. The high efficiency regenerator design can achieve complete regeneration of catalyst with perhaps half the catalyst inventory required in the older regenerators, using a bubbling fluidized bed.
In FCC units, much of the catalyst is lost due to attrition, and an increase in catalyst inventory increases catalyst loss to attrition. Much of the activity loss of the FCC catalyst is due to steaming in the regenerator. This steaming is not intentional, but most regenerators operate with 5-10 psia steam partial pressure (due to entrained stripping steam, and water of combustion). Thus the regenerator is not only a regenerator, it is a catalyst steamer, and deactivator. Increased catalyst inventory in the regenerator leads to increased steaming and deactivation of the FCC catalyst.
There is therefore a great incentive to do everything possible to reduce the catalyst inventory of a regenerator, and to improve the efficiency of regenerator. That is why a majority of new FCC construction uses the high efficiency regenerator design.
Unfortunately, it has not been economically justifiable to convert older style, single dense bed regenerators to the modern H.E.R. design because of the high capital cost associated with simple scrapping the old single bed regenerator. Attempts to simple use the old single stage regenerator as part of a modern two stage, H.E.R. design have not been too successful, as the old single stage units are much larger than either of the beds in an H.E.R. unit. Another complication has been that many of the older units were not designed to operate at the higher temperatures associated with complete CO combustion.
Rather than scrap older FCC regenerators, refiners have tried to improve them, and the FCC process, as much as possible with improvements in catalyst and catalyst additives.
Actually, refiners have known for many years that there were problems with bubbling bed regenerators in general and with the swirl type regenerator in particular. A typical swirl type regenerator is shown in U.S. Pat. No. 3,817,280, which is incorporated herein by reference.
The swirl type regenerator adds spent catalyst to an FCC regenerator having a generally circular cross section. The catalyst is, added via a single inlet, to the dense bed of catalyst in the regenerator in a tangential direction, imparting a swirling motion to the dense bed. The catalyst swirls around roughly 3/4 of the regenerator, and then is withdrawn as regenerated catalyst for reuse in the FCC process.
The swirl regenerator is an elegant concept which causes problems in practical operation. The spent catalyst, laden with coke and poorly stripped hydrocarbons, is added to one portion of the bed. The catalyst removed after the radial traverse of the bed has essentially no unstripped hydrobons, and a very low level of residual coke or carbon on catalyst. For efficient operation, the amount of regeneration gas added should roughly equal the amount of combustible substance to be burned, and this means that very large amounts of combustion air are need where spent catalyst is added, and almost no combustion air is needed where catalyst is withdrawn.
FCC operators have provided means for improving the distribution of combustion air to such regenerators. In U.S. Pat. No. 3,817,280, a better way of controlling the distribution of combustion air was provided. The air distribution grid beneath the bubbling dense bed was radially segmented, and means were provided for adjusting the flow of combustion air to each radial segment. In this way it was possible to fine tune the amount of air added to different radial segments of the bubbling fluidized bed.
The approach of U.S. Pat. No. 3,817,280 provided a better way to distribute the air to a swirl type regenerator. It ignored the problem of inefficiencies regards the distribution of solids to a swirl type regenerator.
In U.S. Pat. No. 3,904,548, which is incorporated herein by reference, recognized the problem of efficient operation of a large size fluidized bed. A baffle was provided, adjacent the tangential catalyst inlet, to mix some regenerated catalyst with incoming stripped catalyst. The baffle provided an expanding annulus of about 20 degrees in the direction of catalyst flow, to prevent undesired catalyst circulation.
The operation of swirl, and other, types of regenerators was significantly improved by the use of CO combustion promoters, discussed hereafter.
U.S. Pat. Nos. 4,072,600 and 4,093,535 teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory. Such combustion promoters improve the rate of CO burning in all types of regenerators, both modern and old. CO combustion promoters help minimize CO emissions, but can cause an increase in the amount of nitrogen oxides (NOx) in the regenerator flue gas. It is difficult in a catalyst regenerator to completely burn coke and CO in the regenerator without increasing the NOx content of the regenerator flue gas. Swirl type regenerators are especially troublesome in this regard, i.e., enough excess air and CO combustion promoter can be added to meet CO limits, but this will greatly increase NOx emissions.
We realized that there was a problem with the basic design of the swirl type regenerator, not so much in the way air was distributed, but with the way the catalyst was distributed. We studied swirl type regenerators, and found that in many units 50% or more of the dense bed of catalyst was relatively stagnant.
We learned that use of a baffle, such as one disclosed in U.S. Pat. No. 3,904,548, did not significantly help reduce stagnant regions in the catalyst bed. The baffle of '548 would help reduce or eliminate bypassing, but would not address the problem of stagnant regions in the catalyst bed.
We also studied cross-flow type regenerators, wherein catalyst is added to one side of the regenerator and withdrawn from a catalyst sink or bathtub on the other side of the regenerator, at the base of the regenerator. Although the flows in the cross-flow type regenerators are quite different from those in the swirl type regenerator, both suffer from the same problem, namely that too much of the bed is stagnant.
We have discovered a way to overcome many or the deficiencies of stagnant regions in these large, bubbling bed regenerators by making changes in the way that catalyst was withdrawn.